1. Field of the Invention
The present invention relates to a legged robot having a trunk link and a pair of legs connected to the trunk link. The legged robot is sometimes termed simply “robot” herein.
2. Description of the Related Art
Legged robots having a pair of legs connected to a trunk link are being developed. Each leg has a plurality of links. Furthermore, a plurality of joints is disposed in each leg. Each joint connects at least two links, and rotates the connected links relative to each other by using the power that is supplied thereto. In some cases, the links connected by the joints are links composing a leg, while in other cases, they consist of the trunk link and a link composing a leg. The joint that connects the trunk link and the link composing a leg is termed a “hip joint”. Herein, a joint that rotates the connected links relative to each other with one degree of freedom is represented as one single joint, while a joint that rotates the connected links relative to each other with two degrees of freedom is represented as two independent joints.
A legged robot walks by alternately swinging each of its pair of legs along forward and backward direction with respect to the trunk link, via appropriate control of the power that relatively rotates the links that are connected by the joint. The motion of the leg swinging forward and backward is mainly realized by the motion of a joint that rotates the connected links in a plane that crosses a line extending in the lateral direction of the robot and that has its rotation center located at position that is most distant from the floor (that is, in the highest position).
Other than a rotary joint having a physical rotation shaft, the joint that rotates links relatively with respect to each other may be realized as a sliding joint having the following configuration. The sliding joint has a rail that extends along an arc with an imaginary rotation axis as its center, and a sliding member that moves on such rail. The sliding joint has no physical rotation shaft, but has a rotation axis that serves as the center of rotation. As used herein, the “rotation axis” of a joint signifies the central axis of rotation, and is not restricted to a form of a physical shaft. Also, the “rotation center” is the central point, in a plane defined by links rotating relatively to each other, of the relative rotation of such links. In other words, the rotation center means the cross point of the rotation axis and the plane formed by the relatively rotating links.
A joint that rotates connected links in a plane that crosses a line extending in the lateral direction of the robot is termed a “pitch joint” herein. Expressed differently, a pitch joint is a joint that rotates connected links around a rotation axis extending in the lateral direction of the robot. Also, the pitch joint whose rotation center is located in the position highest from the floor is termed the “uppermost pitch joint”.
Japanese Patent Application Publication No. 2005-186650 (Patent Document 1) discloses a legged robot having a pair of legs. This legged robot is provided with a cabin in which a person rides, and a pair of leg links. The top ends of the leg links are connected to the bottom of the cabin by means of pitch joints. More precisely, with this legged robot, the uppermost pitch joint is disposed at the top end of each leg, beneath the cabin. The cabin in Patent Document 1 corresponds to the trunk link herein, and the leg links correspond to the legs herein.